Combat Pragmatist/Fan Works

In Twilight Valley everyone fights dirty. It even gets a Lampshade Hanging.
The Firefly fanfic Forward also plays with this, including replicating the legendary ''Raiders of the Lost Ark scene with River casually shooting an Arrogant Kung Fu Guy who tries to fight her with a bo staff. At one point in the most recent story arc, Mal points out that he considers any fight where he's forced to fight fair as inherently unfair.
More or less everybody in Aeon Natum Engel, especially the Replica Elite.
In Urusei Yatsura The Senior Year, during part 10, an OC gives this advice:

"Now, here's something I once heard when it comes to a fight! You kick them in the balls, stab them in the back, poke their eyes out, and if they're still in the mood to fight..." she gives them a fanatic sneer, "...THEN, you fight dirty!"

- And a few paragraphs later...

Mie then transforms into a female General Patton. "Then go do it!" she points to the door with her riding crop. "Remember, you can't serve your country by dying for your country! You serve your country by making the other dumb bastard die for his country!"

The Basalt City Chronicles has Tors Beers (who hates fighting) taught to end fights as quickly as possible--biting, clawing, breaking bones, and outright killing are all acceptable means to win (though it's best to avoid the last).
The Battle Fantasia Project gives us Akiko Yamaguchi, aka Magical Girl Star Reverie, whose Mentor Mascot was killed in the line of fire before she could receive one of several power-ups needed to fight her latest batch of villains. By the time the next batch of villains has come around, Akiko is skipping her Finishing Move and using gasoline drums and Car Fu alongside numerous other tricks instead of more traditional magical girl tactics.
In The Man With No Name, Mal is quick to try just shooting the villain once things go to hell. It doesn't work, but hey, he tried.
Kaoru from the Rurouni Kenshin fanfic Frozen Moonlight is quite willing to let her opponents think she's helpless while hiding the knife taped to her wrist or to try bashing their skulls in during their monologues. Notably, her father specifically taught her to think like this and to use other people's perceptions of her being vulnerable to her advantage.
Keleria in Children of the Stars, messy kills when available, groin attacks, biting, she gets progressively worse the more she slips into that blood rage thing.
Attelus Kaltos the main character of Secret War one of his first actions in the story is to kick an attacker in the shin...With a boot knife.
In Queen of All Oni, one of Jade's Co-Dragons, Left, comes off as this. While he -- like his "brother", Right -- usually fights with his swords, he'll resort to quicker methods if necessary. For example, during the assault on Lung's fortress, he breaks out a chainsaw to get by a Hybrid Monster, and when confronted with a gateway that's impervious to magical attack, he simply blows it open with dynamite.
- Captain Black's Number Two, Agent Wisker, also qualifies. When Jade infiltrates Section 13 to try and steal the masks in the Vault, he stands in the way. She attacks him with magic, and he retaliates by shooting her -- intentionally non-lethal shots, and it doesn't even work, but it's still the thought that counts.
- Jade's foreman Blankman -- we finally get to see him in combat when he fights Drago, and while he primarily fights with savate (a French martial art), he does enhance it with magic. And when even that isn't enough to win, he resorts to whipping out a mystically-enhanced shotgun.
In Ace Combat: The Equestrian War, Gilda uses Medley as a living shield to defend herself against Rainbow Dash's attempts to fight her. She breaks Medley's wings anyway.
Doug Sangnoir of Drunkard's Walk is very much the kind of fighter who feels the only fair fight is one where he has an overwhelming advantage. Then again, this appears to be a key attribute of the superteam to which he belongs in his home universe:

I don't like even odds. They mean the good guys lose half the time. One reason the Warriors are as successful as we are is that in any given opportunity, we will field far more force that is far nastier than the enemy is prepared to deal with. We don't fight just to win. We fight to crush the enemy utterly. We fight to overwhelm and destroy.

He frequently advises those he trains -- from Harry Potter to Sailor Moon -- to never leave a defeated enemy alive to attack them again.